Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Tunable Three-Dimensional Nanostructured Conductive Polymer Hydrogels for Energy-Storage Applications Chunying Yang,† Pengfei Zhang,† Amit Nautiyal,‡ Shihua Li,† Na Liu,† Jialin Yin,† Kuilin Deng,*,† and Xinyu Zhang*,†,‡ †
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Analytical Science and Technology Laboratory of Hebei Province, College of Chemistry & Environmental Science, Hebei University, Baoding, 071002 Hebei, China ‡ Department of Chemical Engineering, Auburn University, Auburn 36849, United States S Supporting Information *
ABSTRACT: Three-dimensional (3D) nanostructured conducting polymer hydrogels represent a group of highperformance electrochemical energy-storage materials. Here, we demonstrate a molecular self-assembly approach toward controlled synthesis of nanostructured polypyrrole (PPy) conducting hydrogels, which was “cross-linked” by a conjugated dopant molecule trypan blue (TB) to form a 3D network with controlled morphology. The protonated TB by ion bonding aligns the free sulfonic acid groups into a certain spatial structure. The sulfonic acid group and the PPy chain are arranged by a self-sorting mechanism to form a PPy nanofiber structure by electrostatic interaction and hydrogen bonding. It is found that PPy hydrogels doped with varying dopant concentrations and changing dopant molecules exhibited controllable morphology and tunable electrochemical properties. In addition, the conjugated TB dopants promoted interchain charge transport, resulting in higher electrical conductivity (3.3 S/ cm) and pseudocapacitance for the TB-doped PPy, compared with PPy synthesized without TB. When used as supercapacitor electrodes, the TB-doped PPy hydrogel reaches maximal specific capacitance of 649 F/g at the current density 1 A/g. The result shows that PPy nanostructured hydrogels can be tuned for potential applications in next-generation energy-storage materials. KEYWORDS: conducting polymer hydrogel, self-assembly, 3D nanostructure, trypan blue, energy storage
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INTRODUCTION Hydrogels are polymer networks where polymer chains are held together by cross-links. They have a high degree of hydration and a solid three-dimensional (3D) network resulting from cross-linked chains. They are highly flexible because of their high water content. Conducting polymer hydrogel (CPH) is a group of materials that possess both electrical conductivity and flexibility, which overcomes the intrinsic high rigidity of the conjugated system in conducting polymers, and the hydrophobicity caused by the π−π stacking of the chain.1−3 The synergy between conducting polymers and hydrogels combines the properties of the organic conductor and 3D network hydrogel.4−6 The 3D conducting framework of CPH promotes the charge carrier transportation through the network.7−9 Additionally, CPHs provide excellent interface between various systems, such as an electrode and an electrolyte, biological and synthetic systems, which offers them a great potential for various application areas, such as electrode materials for energy-storage devices, biofuel cells, and bioelectronics.10−13 Previous reported conducting hydrogels were interconnected nanofibers of conducting polymers that exhibit poor mechanical properties, limiting their applications in flexible electronics. On another aspect, conductive networks © XXXX American Chemical Society
composed of conducting polymers and polyacrylic acid (PAA) hydrogel showed high mechanical strength.14 However, using PAA, their limited mass loading of active materials (conducting polymer) results in a low electrochemical performance. This route to manufacture conductive hydrogels is known as hybrid systems, where prefabricated hydrogels are used as scaffolds for the conducting polymer to grow.15−18 Another way to synthesize hydrogel is the continuous phase, where only the conducting polymer is used as a 3D network using some crosslinking agent or gelator.15,19 The hydrogel is formed by the self-assembly of conducting polymer chains. In previous reported work, researchers typically used phytic acid as a dopant and a gelator to form a conducting hydrogel. However, their electrochemical performance is not as high as expected because of the insulating property of phytic acid.16,20 This selfassembly of polymer chains can provide an effective method for the controlled synthesis of polymer hydrogels through noncovalent interactions. It provides the smallest one-dimenReceived: November 1, 2018 Accepted: January 8, 2019 Published: January 8, 2019 A
DOI: 10.1021/acsami.8b19180 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Mechanism for synthesis of PPy hydrogel-TB (PPy-TB). The TB molecule self-assembles into an ordered 1D nanostructure by selfsorting mechanism and spatial effect.
are also obtained. Moreover, because the TB molecule is a good organic semiconductor, the dopant TB enhances the interchain charge transfer of PPy, resulting in a great improvement of the conductivity and electrochemical capacitance. In addition, the hydrogel is grown on carbon cloth without the need for additional binders, which leads to good mechanical adhesion and ductility, and establishes strong electronic transmission pathway between PPy-TB and carbon cloth, the current collector.43 With a specific capacitance of 649 F/g, PPy-TB shows a great potential for energy-storage applications.
sional (1D) nanofibrous structure that provides efficient charge and energy transfer.21−23 Nanomaterials with a unique structural stability have important research implications for enhancing the capacitive properties of conductive hydrogels,24−27 and the application of structurally related properties involving such materials requires scalable and efficient morphologically tunable synthesis methods.28−30 In recent years, disc-shaped phthalocyanine molecules have been coupled to various metal elements that can be used as dopants and can self-assemble into highly oriented and predictable features.31−35 They showed the morphological control of polypyrrole (PPy) hydrogels,36 and promoted synergistic effects to enhance both the mechanical and electrical properties. Li and co-workers used copper(II) phthalocyanine tetrasulfonate salts as a cross-linking agent to prepare a tunable 3D nanostructured PPy hydrogel framework that was used for Li-ion battery electrodes.37 Metal−ligand and conducting polymer network were used to assemble selfhealing conducting hydrogel.38 Yang and co-workers used ironbased tetrakis(4-carboxyphenyl)porphyrin self-assembled PPy hydrogel for water splitting.39 However, these molecules have limited application because of high cost and tedious synthesis process.34,40 Therefore, the discovery of new dopants is very essential in terms of enhancement of electrical conductivity and bridging the conducting polymer networks. At the same time, the functionalization of CPH backbones will be able to induce unexpected properties.41 In this work, we introduced a liquid-crystal-conjugated molecule; trypan blue (TB) contains numerous hydrophilic groups that can be used as a dopant and a gelling agent, through a static in situ polymerization of pyrrole monomers. This leads to self-assembled nanostructured PPy hydrogel with controlled morphology. The gelation process occurs simultaneously with the polymerization process. The amino group in the TB molecule is protonated in the acidic ammonium persulfate (APS) aqueous solution, and the partially protonated TB molecule is ionically bonded to the sulfonic acid groups on other TB molecules. The free sulfonate ion forms a certain spatial structure through a self-sequencing mechanism, which interacts with the PPy chain through ionic and hydrogen bonding, and the PPy chains were oriented through a selfsorting mechanism. As shown in Figure 1, PPy nanofibers are formed through the proposed mechanism. Changing the dopant concentration can form nanofiber structures with different morphological sizes. Compared with PPy which tends to grow into nanoparticles,42 polyaniline (PANI) is beneficial to its growth into a fibrous structure.17 In a series of experiments, PANI nanofibers with controlled morphology
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EXPERIMENTAL SECTION
Synthesis. According to the previous report,36 1.2 mmol of pyrrole (99%, Aladdin) was dissolved in 1 mL of isopropanol to prepare solution A. TB (0.03 mmol ; 60%, J&K) was dissolved in 2 mL of deionized water to prepare a dopant solution, and then APS is dissolved in the dopant solution to obtain solution B. The two solutions were cooled to 4 °C and then mixed together, resulting in the formation of a black gel, which indicates the formation of PPy hydrogel. Further purification of the PPy hydrogel was carried out after 12 h reaction, using deionized water dialysis for 24 h, to remove impurities from the reaction system. Other dopants that were used for comparison include direct blue 2 (0.04 mmol, DB2) (TCI) and direct violet 1 (0.06 mmol, DV1) (TCI), which have three and two sulfonic acid functional groups, respectively. The same method was used to synthesize other conductive polymers, including PANI, poly 3,4ethylene dioxythiophene (PEDOT) (using aniline (99%, J&K), and EDOT (99%, Aladdin), respectively). In addition, PPy was synthesized in water as a blank control, using APS as an oxidant. Electrode Preparation. The carbon cloth (1 cm × 2 cm), used as a current collector, was washed with deionized water and ethanol to remove impurities. This was followed by treatment with concentrated HNO3 for 8 h at room temperature. The treated carbon cloth was then washed with deionized water and ethanol again, dried in a vacuum oven at 60 °C for 30 min. The carbon cloth (1 cm × 1 cm) was dipped into the reaction mixture, immediately after mixing solutions A and B. After 12 h of reaction, the PPy hydrogel-coated carbon cloth was purified by deionized water for 24 h for the electrochemical test. The galvanostatic current charge and discharge tests were carried out after vacuum drying at 50 °C. Using the following equation, specific capacitance was calculated according to the galvanostatic charge/discharge curve Cs =
I ·Δt m·ΔV
(1)
where Cs is the specific capacitance, I is the current, Δt is the discharging time, ΔV is the potential window, and m is mass of the electroactive material. Similarly, according to eq 2, the area of the cyclic voltammetry (CV) curve is used to calculate the Cs value B
DOI: 10.1021/acsami.8b19180 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Cs =
PPy and PPy with different dopant molecules (DB2 and DV1) (Figure S1). X-ray Diffraction. The crystalline structure of PPy and PPy-TB hydrogels was investigated by XRD, as shown in Figure 3. Sharp peaks can be observed for TB at 10°, 15°, 20°,
∫ I dV 2m·v ·ΔV
(2)
where I is the current density, m is mass of the electroactive material, v is the scan rate, and ΔV represents the scanned potential window. Characterization. Scanning electron microscopy (SEM) (JSM7500, Hitachi) is used to observe microscopic characteristics of samples. Powder X-ray diffraction (XRD) from Bruker’s D8 ADVANCE was used to analyze the crystal morphology. The Quantachrome Autosorb iQ apparatus was used for N2 adsorption/ desorption experiments. An Elemental Analyzer (CE-440, Exeter Analytical, Inc, USA) was used to determine the element ratio. The Fourier transform infrared (FT-IR) spectrometer, used for determining the chemical structure of the sample, was Nicolet iS10 from Thermo Fisher. The pyrolysis behavior of different samples was studied using a thermogravimetric analyzer (TGA 4000, PerkinElmer) in N2 at a heating rate of 10 °C min−1 from 40 to 800 °C. Electrochemical testing including CV and electrochemical impedance spectroscopy (EIS) were carried out with electrochemical workstation (CHI660E, Shanghai) in a three-electrode system. The reference electrode was Ag/AgCl, the counter electrode was platinum wire, and the electrolyte was 1 M H2SO4 aqueous solution. Conductivity is measured using a semiconductor powder resistivity tester (ST-2722, Suzhou Jingge).
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Figure 3. Comparison of XRD patterns of PPy-TB, PPy, and TB.
RESULTS AND DISCUSSION Fourier Transform Infrared. The structural properties of CPHs were investigated by FT-IR, as shown in Figure 2. The
21°, 30°, and 47°. The peaks associated with crystalline TB disappeared in PPy-TB, indicating that the crystal structures of TB undergo an oxidation reaction after in situ polymerization and a recombination of TB molecule occurred in the PPy matrix.37 The broad peak around 22° for PPy, and the peak between 20° and 30° for PPy-TB nanocomposite could be assigned to the π−π stacking of the aromatic rings of PPy. This could serve as an indication of well-packed PPy along the polymer backbone.46 Morphology. Through SEM, TB-doped PPy tends to form interconnected nanoscale conductive matrix nanofibers rather than agglomerated bundles, which greatly increases the specific surface area and enhances the charge carriers’ transport properties. The morphology of PPy hydrogels reveals that the concentration of TB plays an integral role in PPy selfassembly. As TB concentration decreases, the diameter of PPy nanofibers gradually decreases, that is, the lowest average diameter (57 nm) was obtained at 0.001875 M TB. However, further reducing the TB concentration to 0.0009375 M, much higher nanofiber diameter (∼110 nm) was obtained with some granular particles on the nanofiber surface, indicating that the TB concentration affects the interaction between the TB molecules (Figure 4 inset). Because of the strong molecular interaction between CPH and TB, the PPy-TB hydrogel establishes a uniform and interconnected 3D network, with possibly better mechanical properties. As a result, it reduces the agglomeration of fiber significantly and made PPy nanofibers more regularly oriented. This is due to the fact that TB facilitates more spatially tight alignment between the molecules. In contrast, the morphology of the PPy without TB is not a nanofiber because of a possible side-chain formation and cross-linking along PPy chains, which interferes ordered packing for PPy nanofiber formation.47,48 In addition, spatial structures between TB molecules may prevent cross-linking among PPy backbones and promote the nanofiber growth. Therefore, TB molecules play a significant role in PPy self-assembly and its morphological regulation. In fact, the self-sorting mechanism of TB molecules allows free sulfonic acid groups to be spatially ordered and arranged to make PPy chains to grow to one-dimensional nanostructures.
Figure 2. FT-IR spectra of PPy-TB-doped hydrogels and PPy without TB.
PPy control (without TB) shows characteristic peaks as listed below: in-ring stretching of CC bonds (1552 cm−1), ringstretching mode (1340 cm−1),44 in-plane deformation of N−H bond (1045 cm−1),38 C−C stretching (1297 cm−1), in-plane deformation of the C−H bond and N−H bond (1042 cm−1),45 in-plane bending absorption peak of the CN bond (1548 cm−1), and stretching vibrations of CN+ bonds (1174 cm−1) and C−N+−C bonds (903 cm−1), respectively.39 All of the peaks mentioned above can be found in TB-doped PPy, which indicates the formation of PPy-TB nanocomposites. Compared to the PPy control sample, the characteristic peaks of TBdoped PPy slightly shifted to a lower wavenumber (red shift), which indicates that the conjugated TB structures promoted delocalization of the π-electrons from PPy backbones.38 In addition, we also found that there are significant red shifts in the stretching vibration peaks of CN+ and C−N+−C functional groups in different concentrations of TB-doped C
DOI: 10.1021/acsami.8b19180 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. SEM images of PPy with varied concentrations of TB: (a) 0.015 M TB (PPy-TB1); (b) 0.0075 M TB (PPy-TB2); (c) 0.00325 M TB (PPy-TB3); (d) 0.001875 M TB (PPy-TB4); (e) 0.0009375 M TB (PPy-TB5); and (f) no TB (PPy). (Inset: Average diameters of nanofibers of PPy hydrogels).
Figure 5. SEM images of PPy doped with different dopants: (a) PPy-TB4; (b) PPy-DB2; and (c) PPy-DV1 with their corresponding structures. Molecular structure of (d) TB, (e) DB2 and (f) DV1; 3D model of molecular structures (g) TB, (h) DB2, and (i) DV1,.
This general synthetic route can be extended to the synthesis of PANI hydrogels (Figure S2) with interconnected nanofiber morphology and with morphological adjustability, which indicates that the formation of nanofibers between TB molecules and polymers is mainly because of the hydrogen bonding effect. However, when applying this synthetic method to polymerize EDOT, it did not give homogeneous nanofiber structures. It is probably because of the difficulty to form hydrogen bonds between TB molecules and PEDOT because of the limited water solubility of EDOT monomers. To better understand the dopant effects on PPy selfassembly, we selected DB2 and DV1 as control dopants, with tribasic sulfonic acid and two sulfonic acid groups (Figure 5e, f). DB2 and DV1 have molecular formulas similar to that of TB
(Figure 5d) but exhibit completely different spatial structures, as shown in Figure 5g−i. The structural morphology of the PPy hydrogel is determined by both the sulfonic acid groups they have and the structure of TB, DB2, and DV1. In Figure 5a−c, the morphology of PPy hydrogels synthesized by TB, DB2, and DV1 doping is nanofibrous, short particle necklaces, and particles, respectively. When using TB with the tetrasulfonate group as dopants, smooth PPy nanofibers were obtained, whereas DB2 with trisulfonate group yielded agglomerated nanofibers with granules on the nanofiber surface. It was observed from SEM images that DB2 is still conducive to the arrangement of PPy chains, which indicates that it can arrange the free sulfonic acid groups in space through the self-sorting mechanism and form PPy 1D D
DOI: 10.1021/acsami.8b19180 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a) Nitrogen adsorption/desorption isotherms of PPy hydrogels, (b) specific surface area of PPy and PPy-TB1-5, (c) pore volume of PPy and PPy-TB1-5, and (d) pore size of PPy and PPy-TB1-5.
Table 1. Conductivity of Different Samples PPy-TB1
PPy-TB2
PPy-TB3
PPy-TB4
PPy-TB5
PPy-DB2
PPy-DV1
0.42 S/cm
1.33 S/cm
2.07 S/cm
3.3 S/cm
1.25 S/cm
1.05 S/cm
0.58 S/cm
TB and PPy can be seen at around 100 °C. At this temperature, the loss of water in PPy and PPy-TB was relatively severe, whereas the temperature was stable at about 100−250 °C and their no significant weight loss. The decomposition of PPy and PPy-TB started around 280 °C, and the decomposition is not sharp. In contrast, the PPy material incorporating TB lost mass after 280 °C, but the mass decreased slowly compared to PPy, indicating an interaction between TB and PPy. The interaction between TB and PPy increases the structural stability of PPy-TB, resulting in an increase in thermal stability. That is why PPy-TB has a higher residue weight than PPy at 800 °C. The conductivity of synthesized samples was measured by a semiconductor resistivity of the powder tester and was tabulated in Table 1. This conductivity increases greatly compared to the pristine PPy (0.07 S/cm) conductivity reported in the previous literature.36 The results confirm that the increase in polymer conductivity is related to the improved ordering of unidirectional polymer chains, and TB dopant molecules are beneficial to enhance the interchain charge transfer between conductive polymer chains.57 The higher conductivity of PPy-TB is related to the fact that TB is a conjugated semiconducting molecule, and TB dopant molecules have a certain synergistic effect on the conductivity of PPy hydrogels.58 The conductivity of DB2 and DV1 as dopant molecules was measured as 0.5 and 1 S/cm, respectively; they did not form smooth fibers. This likely blocked the charge transfer between the chains, resulting in a decrease in their electrical conductivity. CV and EIS were used to analyze the electrochemical properties of the TB-doped PPy hydrogels. CV (cycle 30) and EIS tests were conducted in 1 M aq H2SO4. CV plots of all the
nanostructures through spatial effects. On the other hand, when PPy was doped using DV1, it formed granular nanostructure similar to that of typical PPy morphology, which implies that DV1 behaves very differently in terms of templating PPy self-assembly, compared to TB. Porosity and specific surface area are two key factors determining the energy-storage properties of nanomaterials. To further understand the regularity and the orientation of PPy-TB nanofibers, N2 adsorption/desorption isotherms were measured, and a type IV isotherm was obtained, as shown in Figure 6a.49−53 The adsorption characteristics are fairly low at P/P0 lower than 0.9 and there is no obvious change related to the adsorption of N2 in micropores (
